iommu: Ensure .iotlb_sync is called correctly

Many drivers have no reason to use the iotlb_gather mechanism, but do
still depend on .iotlb_sync being called to properly complete an unmap.
Since the core code is now relying on the gather to detect when there
is legitimately something to sync, it should also take care of encoding
a successful unmap when the driver does not touch the gather itself.

Fixes: 90c5def10bea ("iommu: Do not call drivers for empty gathers")
Reported-by: Jon Hunter <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Robin Murphy <[e-mail]>
Tested-by: Jon Hunter <[e-mail]>
Reviewed-by: Jason Gunthorpe <[e-mail]>
Tested-by: Russell King (Oracle) <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

drivers/iommu/iommu.c

···
 
 		pr_debug("unmapped: iova 0x%lx size 0x%zx\n",
 			 iova, unmapped_page);
+		/*
+		 * If the driver itself isn't using the gather, make sure
+		 * it looks non-empty so iotlb_sync will still be called.
+		 */
+		if (iotlb_gather->start >= iotlb_gather->end)
+			iommu_iotlb_gather_add_range(iotlb_gather, iova, size);
 
 		iova += unmapped_page;
 		unmapped += unmapped_page;